Measurement of blood flow in the heart and vessels using the Doppler effect is well known. Whereas the amplitude of the reflected waves is employed to produce black and white images of the tissues, the frequency shift of the reflected waves may be used to measure the velocity of reflecting scatterers from tissue or blood. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over the black and white anatomical image. The measured velocity of flow at each pixel determines its color.
Most ultrasound color flow systems determine blood velocity in the body by measuring the phase change caused by the Doppler shift of the returning backscattered ultrasound signal. This phase change is usually determined by firing multiple pulses in the same direction, and for each range cell, high pass filtering the data to remove static tissue components and correlating adjacent firings. The phase of the complex correlation function produces the mean velocity. This method has several limitations. The velocity sensitivity is primarily determined by the pulse repetition frequency (PRF). When imaging low flow, a low PRF is necessary to allow a measurable phase change to take place between firings. This is done at the expense of frame rate, since it takes longer to make each ultrasound image.
A conventional technique is to interleave firings from several directions, so that the effective PRF is lowered while the actual PRF remains high, allowing for acceptable frame rates to be maintained. The low-flow sensitivity inherent in the interleaving technique, however, is achieved at the expense of high-velocity resolution. If the phase change between effective firings is greater than plus or minus .pi., velocity ambiguities occur which cause the estimates to alias, yielding erroneous results.
A technique used in Doppler weather radar called staggered PRF is often proposed to alleviate the aliasing problem. A firing sequence involving several different long pulse repetition intervals is used, and the resulting estimates can be unfolded to allow velocity resolution in a range that exceeds the aliasing limits of both of the PRF intervals.
The staggered PRF method is difficult to implement in medical ultrasound applications because it is very difficult to design high-pass wall filters with the right characteristics for nonuniformly spaced data points. In order to obtain accurate mean velocity estimates, the filters must be linear in phase and have a uniform frequency sensitivity over the entire passband.